The present invention relates to a silver halide emulsion suitable for incorporation in a silver halide photographic material. More particularly, the invention relates to a process for producing a silver halide photographic emulsion comprising monodisperse twinned grains.
With the recent tendency to use color negative films with high sensitivity and in a small format, the demand for silver halide photographic materials capable of producing images of high quality is becoming more stringent than before. With a view to improving the granularity of monodisperse, normal crystalline silver halide grains in emulsions, many approaches have been proposed for controlling the grain size, size distribution, the halide composition within the grains, and their crystalline structures. On the other hand, polydisperse silver iodobromide twinned grains are conventionally used to prepare emulsions adapted to high-sensitivity photographic films. The exact reason why emulsions comprising twinned crystals provide a higher sensitivity is not clear but the primary reason would be that twinned crystals have a propensity to grow to large sizes. Additionally, the twinning planes within silver halide grains are considered to play an important role during the photographic process.
While twinned crystals have advantageous photographic properties and are extensively used in emulsions, the mechanism of their formation has not been fully unravelled and no technique has been established that is capable of satisfactory control over their growth.
Japanese patent publication No. 58-36762 and Unexamined Published Japanese patent application No. 52-153428 proposed techniques for controlling the growth of monodisperse twinned crystals so that they acquire advantageous photographic properties, but the obtained twinned crystals do not have a completely satisfactory level of monodispersity. Unexamined Published Japanese patent application Nos. 55-142329, 58-211143 and 58-209730 disclose growth methods for monodisperse silver halide crystals, but the emulsions obtained by these methods have such a small fraction of twinned crystals that they can hardly be described as emulsions comprising monodisperse twinned grains.
It has been predicted theoretically that by narrowing the size distribution of the grains in a silver halide emulsion, the efficiency of grain utilization is increased (i.e., "dead grains" are decreased) and a higher sensitivity and better granularity are provided. However, no emulsion has ever been prepared that comprises satisfactorily monodisperse twinned crystals.